21 patients were enrolled into the study. Patients were randomized in a double-blind fashion to receive either placebo or magnesium sulfate infusion. We have confirmed that patients with congestive heart failure are profoundly deficient in their intracellular magnesium concentrations. These levels are transiently normalized by infusion of 12.5 grams of intravenous magnesium sulfate. The mean concentrations at twent-four hours are within the normal limits. At forty-eight hours there is a mild decrease in the mean magnesium concentrations. At one week, the mean levels are similar to baseline. There is a temporal improvement in premature ventricular contractions, as measured by Holter monitoring, which correlates with intracellular magnesium concentrations. A series of interim statistical analyses were done on the relationship between intracellular magnesium concentration and repolarization parameters in fifteen patients. There is a trend that approaches sstatistical significance between a rise in intracellular magnesium concentrations and a decrease in the QT variability index (approaching the normal limits). We hope that as we accumulate more data, this relationship will become statistically significant. In addition, we have evaluated left ventricular contractility before and after study drug infusion, using a novel technique established by David Kass, M.D. This portion of the study remains blinded, however our hypotesis is that contractility will improve in subjects receiving magnesium.